wis welcome to the Forum!
If you are familar with the Home (free) version of Avast! try going through your settings and make sure you have everything configured as before.
START>PROGRAMS>AVAST! ANTIVIRUS. Then select SETTINGS;from the simple user interface check through and see they are o.k.
Go to UPDATE (BASIC) and select AUTOMATIC.
It's advisable to have Automatic Updates 'Enabled' .
From reading your post it sounds that maybe you are 'Manually' updating, which should be O.K. but could be where your problem lies.
Also, as a quick remedy, try going to ADD/REMOVE programs in Windows CONTROL PANEL and selecting Avast!, then select Option> REPAIR. This will re-install Avast! and update in case there's been an error with your installation.